1. Field of the Invention
The present invention relates to a speed detection device and a servomotor.
2. Related Art
In the motion control of servomotors, a position detector outputs a sine signal and a cosine signal based on the moving distance of a machine or on the moving angle of a shaft. An arithmetic operation circuit of the position detector makes samplings of sine signals and cosine signals every unit time and calculates a moving speed or an angular speed from a difference of distances or angles between two samples and from a time interval of the sampling.
As shown in, for example, FIGS. 8A and 8B, when coordinates obtained by samplings carried out twice are shown by (x1, y1) and (x2, y2) and the time interval of sampling is shown by t(sec), the angular speed ω (rad/sec) is shown by Expression 1. “sec” means second and “rad” means radian.ω=(θ2−θ1)/t=(tan−1(y2/x2)−tan−1(y1/x1))/t  (Expression 1)
Here, θ1(rad) and θ2(rad) are absolute angles (deflection angles) at the times of the two samplings. “tan” means tangent. As shown in Expression 1, to obtain the angular speed ω, the deflection angles must be calculated from the coordinates (“Facts of Logic and Design of AC Servo System” (“AC servo system no riron to sekkei no jissai”), Hidehiko Sugimoto et al, p 142-p 149, Sogou Densi Shuppan Corp.)).
To cope with the above problem, it is contemplated to use a CPU or a custom LSI (Large-Scale Integrated circuit) as an arithmetic operation circuit and to realize the operation of Expression 1 by a digital logic circuit.
However, the arctangent (tan−1) requires division of sine/cosine. The following two problems arise in the calculation of the arctangent. One of the problems resides in that the absolute value of the value of a tangent (cosine/sine) greatly increases as a cosine signal located on a unit circle more approaches to zero. As a result, the error resulting from the arctangent greatly increases.
The other of the problems resides in that the division (cosine/sine) is necessary to obtain an arctangent and a tangent. Division is a circuit which cannot be easily realized by a digital circuit, and thus a large-scale digital circuit must be used to realize division as compared with multiplication and addition.
Accordingly, there are desired a speed detection circuit and a servomotor that can detect a moving speed or an angular speed by a small arithmetic operation circuit with pinpoint accuracy.